A method is described in GB 1,490,535 whereby a press serves to form and harden a relatively thin steel plate with good dimensional stability. The steel plate is heated to a temperature above its AC3 point and is then pressed in less than 5 seconds into the final shape between two indirectly cooled dies under substantial shape change and is subjected to a rapid cooling while remaining in the press, so that a martensitic and/or bainitic fine grain structure is achieved. A hot-shaped part produced this way is used in motor-vehicle construction for example for structural and safety parts such as bumpers and B columns.
Currently commercial hydraulic presses are used for hot shaping and press hardening, and are modified and adjusted for this special task. As described in above-cited GB 1,490,535 these dies are cooled indirectly. For this purpose cooling passages are set about 5 mm under the die surface alongside the die inner side with the given component contour. A liquid coolant, typically water, is pumped through these cooling passages to transfer to the outside heat passed from the hot component to the die. The cooling passages are formed in the die by drilling. Thus, the placement of the drills can be carried out only in sections, especially with a nonlinear shape, as the drilled holes must be straight. With a curved and/or complex component shape the die is hence separated into many single segments in which cooling passages are placed in sections and that then has to be connected with the other sections through a further passage or has to open to the outside, Consequently, the production of such indirectly cooled dies is laborious and expensive. In addition, leaks caused by tension cracks occur in the die during operation, so that cooling liquid leaks from the cooling passages and evaporates. Hence, the dies have to be repaired continuously, creating always down times in production arise.
A method is described in EP 1 191 111 of Gustavsson whereby a flat metal component is hardened by first heating it, and then conveying it along a conveying path to a position between an upper and a lower die of a press. Then the two parts are pressed together to deform the part and at the same time these parts are cooled. The cooling can be done by actually lowering the parts into a bath of the cooling liquid while they grip the deformed workpiece. Here the workpiece, however, is deformed in advance and not in the press that basically serves as a holder for maintaining workpiece shape during tempering.
In DE 26 03 618 of Kenebuc, a method and apparatus are described for heat treatment of steel where a press is used having shaped dies in whose faces concentric annular grooves are formed. Coolant passages extend through the dies at a spacing from their working faces. A cooling liquid is fed through these passages to up to the annular grooves for direct contact with a clamped workpiece to be hardened. In this manner a continuous hardening is guaranteed. This method is relatively slow and uses dies that are very complicated and hard to maintain.